From the technical article "AVLS--A system for automatically determining the position of vehicles" by A. Bethmann et al., 1994, Alcatel SEL, Stuttgart, it is known to use a number of position determination possibilities to compute the exact location of vehicles. The GPS Global Positioning System is used to that end for example. The system comprises 25 satellites, not all of which are still active however. The accuracy of the location is approximately 100 meters for private users. The determination of the position is based on the direct distance measurement between satellites and vehicular receiver by means of the half-wave time delay measurement method with synchronized time reference. Inaccuracies must be expected, because ionospheric disturbances affect the signal propagation time, and atmospheric disturbances and damping limit the availability of the signal. Multipath reception furthermore causes positioning errors, particularly in a city environment. Signal shading is possible in tunnels.
The problems with the global positioning system are partly avoided when a differential method is used. A GPS receiver is installed in a known reference location. This reference receiver is able to determine very accurately the distance from its location to the satellites. In the vehicles these distances are now compared with the distances measured in the vehicles. This determines the errors in the distance measurement and thereby improves the result of the measurement. With this method the positioning accuracy can be increased to 10 meters in the horizontal direction. The above-cited technical article also discusses the type of data transmission between the fixed station and the mobile devices. The use of (PMR=Private Mobile Radio) service radio for communication by the mobile stations with the fixed station is discussed as an example of a solution. Service radio networks are widely used, particularly in local public traffic, but their data capacity cannot be expanded to any desired extent.